The problem is existing repositories have SHA-1 commit hashes, and if you also end up changing those...

There's lots of tools that refer to git commit IDs. Some of those tools may even hardcode a commit ID to be 40 hex digits long. The fact that these tools are external also means that "oh, just rewrite the commit messages or code to refer to the new IDs" isn't feasible. The only way to not break the world is to let people refer to existing commits with their SHA-1 hashes in perpetuity, and it doesn't sound like git is set up to allow this in any way, which means that existing repositories have to stay SHA-1 in perpetuity and that will cause fun down the line if you start having to make SHA-1 and SHA-256 repositories.

Changing from master to main is a one-off change. It might require changing your scripts once to refer to 'origin/main' instead of 'origin/master', but other than that, there is essentially nothing more that needs to be done, there is no risk to historical artifacts that needs to be mitigated.